This appears to be a regression introduced in the most recent Safari Technology Preview (Release 34, 12604.1.27.0.1).
On load, call navigator.mediaDevices.enumerateDevices() and note the device IDs in the source list. Open a new page (leaving the first page open) that also makes an enumerateDevices call. Return to the first page and enumerate your devices again; their IDs will have changed. This appears to occur regardless of whether Page 1 and 2 are on the same or different domains.
The result of this bug is that when Page 1 attempts to then use an input device ID from its first enumeration as a getUserMedia constraint, the call will fail because the device ID is invalid.
navigator.mediaDevices.ondevicechange also not called when these device IDs change as a result of this.
I've tested this issue in Release 38 and it's still happening.
I've got an easy repro test page available here: https://s3.amazonaws.com/safari-devices-testing/index.html; to reproduce:
* open the above page, click "Enumerate devices"; note device IDs
* (optional: click "Enumerate devices" again; note that IDs haven't changed)
* open a second page to the same URL, click "Enumerate devices" there
* return to first page, click "Enumerate devices" again; you'll be prompted to re-authorize microphone access, and you'll be given a new set of device IDs